1. Field of the Invention
The present invention relates to a patch measurement device provided in a printing apparatus, and more particularly to a patch measurement device for detecting the positions of patches constituting a control strip which is printed on printing paper.
2. Related Art Statement
There have conventionally been realized printing apparatuses which incorporate a so-called CTP (Computer To Plate) device, i.e., a prepressing device (=a printing plate recording device) that generates an image on a printing plate based on digital image data. A printing apparatus of this type, referred to as a DI (direct imaging) press, is capable of producing printed materials directly from image data, and therefore may be suitable for producing a variety of printed materials, each in relatively small quantities, over short periods of time. While prepress and other processes in such a digital printing apparatus are automated for ease of operation by non-proficient operators, further automation is desired in the control of ink supply, for example, during a printing process.
The control of ink supply in a conventional printing apparatus is generally realized by means of a separate console-type color measurement device, where a produced sample print is measured on a table. In this case, there is a problem in that a human operator needs to take out sample prints from the printing apparatus as necessary to measure the colors appearing on the printed materials.
In order to reduce the amount of work which requires the presence of a human operator as mentioned above, Japanese Patent No. 2824334 discloses a printing apparatus comprising a means for capturing an image of a printed material. In accordance with this printing apparatus, an image of a printed material is captured on an impression cylinder of the printing apparatus, whereby image data is obtained. This image data is compared against reference image data, which is previously read from a printed material that serves as a control reference, and the amount of supplied ink is controlled based on the comparison result. This printing apparatus has an advantage in that there is no need for a human operator as in the case of employing a console-type color measurement device because the printed material is imaged within the printing apparatus.
However, the aforementioned printing apparatus has a problem in that, since an image of the entire printed material must be read for comparison against the reference image, the size of the image data to be handled becomes large, thus requiring a relatively long image data processing time. Since it is necessary to prepare a reference image, this printing apparatus is not suitable for producing relatively few copies of a variety of printed materials, where agility is of the essence.
In order to solve the above problem, a printing apparatus has been proposed which prints a control strip (other than the actual printing image) on a printed material, such that the control strip is measured within the printing apparatus. FIGS. 9A and 9B are diagrams illustrating specific examples of such control strips. Hereinafter, the details of these control strips will be described with reference to FIGS. 9A and 9B.
FIG. 9A is a diagram illustrating a printed material S which maybe obtained by using the conventional printing apparatus. As shown in FIG. 9A, the conventional printing apparatus prints an image im on printing paper, and thereafter prints four control strips cs1 to cs4 and three reference marks rm1 to rm3 on the same printing paper. Hereinafter, such four control strips cs1 to cs4 may collectively be referred to as xe2x80x9ccontrol strips csxe2x80x9d, and the three reference marks rm1 to rm3 as xe2x80x9creference marks rmxe2x80x9d.
The image im is printed on the printing paper, beginning at a position (hereinafter referred to as a xe2x80x9cprint start positionxe2x80x9d) which is located a predetermined gripper margin f away from the leading end of the printing paper. More specifically, the image im is progressively printed in the direction of print progress indicated by the arrow (hereinafter referred to as a xe2x80x9cfirst printing directionxe2x80x9d), beginning from the print start position. The image im has a dimension m along the first printing direction, which is designated according to the image size. The control strips cs and the reference marks rm are printed beginning at a position which is a predetermined distance n away from the trailing end of the image im.
As shown in FIG. 9A, the control strips cs are typically printed on the printing paper with predetermined intervals therebetween along a direction (hereinafter referred to as a xe2x80x9csecond printing directionxe2x80x9d) perpendicular to the first printing direction, and each control strip cs includes a plurality of rectangular-shaped patches arranged in a predetermined order. Each patch may be a half-tone, linework, or solid image which is printed at a predetermined density in a predetermined color. FIG. 9B illustrates an exemplary patch pc1.
As shown in FIG. 9A, the reference mark rm1 is interposed between two adjoining control strips cs2 and cs3. The reference mark rm2 is interposed between the control strips cs1 and cs2, and the reference mark rm3 is interposed between the control strips cs3 and cs4. As such, the reference marks rm1 to rm3 serve as references based on which to detect the positions of the control strips cs1 to cs4. Typically, as exemplified by the reference mark rm 1 shown in FIG. 9B, a reference mark comprises two bars b1 and b2 which run parallel to the first printing direction, and a cross mark c interposed between the bars b1 and b2. Each patch is printed at a position which is predetermined distances awayxe2x80x94along the first and second printing directionsxe2x80x94from a crosspoint P of the cross mark c. For example, the patch pc1 is printed so that the center thereof is at a distance h (along the first printing direction) and at a distance w (along the second printing direction) from the crosspoint P of the reference mark rm1.
An image of the printed material S is captured by an imaging device provided in the printing apparatus, and is passed as xe2x80x9cprinted-image dataxe2x80x9d (i.e., data representing the actually produced printed material) to a patch processing device which is provided in the printing apparatus. Assuming that the patch pc1 is currently to be processed by the patch processing device, the patch processing device first detects the crosspoint P of the reference mark rm1. Furthermore, the patch processing device estimates that a position which is at the patch distance h (along the first printing direction) and at the patch distance w (along the second printing direction) from the detected crosspoint P should be the relative position of the center of the patch pc1, which is currently to be processed. Thereafter, the patch processing device measures the color density information of the patch pc1 at the estimated relative position.
As mentioned above, each control strip cs includes a plurality of patches which are arranged along the second printing direction. In the case where there are fifteen ink keys in the printing apparatus, the total number of patches would be 60 or more. However, due to limited spaces being available for printing the patches pc and the reference marks rm, the total number of reference marks rm which are printed on the printing paper is disproportionately small relative to the large number of patches. Even if an increased number of reference marks rm are employed, it would only invite an increase in the detection frequency of the reference marks rm, thereby resulting in more time being consumed for measuring the color density information. In this respect, the total number of reference marks rm should be minimized. In order to estimate the positions of a plurality of patches composing each control strip cs relative to a corresponding reference mark rm, the printing apparatus employs xe2x80x9cimage-to-print dataxe2x80x9d, i.e., data representing an image to be printed on the printing plate, as theoretical deployment data (theoretical values). In other words, this printing apparatus utilizes theoretical deployment data corresponding to the xe2x80x9cpatch distancesxe2x80x9d to each patch from a corresponding reference mark rm (i.e., patch distances h and w to the patch pc1 as taken from the reference mark rm1) to estimate a plurality of patch positions relative to the reference marks rm in the printed-image data imaged by the imaging device.
However, the aforementioned patch position estimation will become erroneous unless a fixed relationship is maintained between a referential length on the image-to-print data and a referential length on the printed-image data generated by imaging the printed material S which is printed based on the image-to-print data. One cause for such errors is fluctuations in the imaging magnification associated with the imaging device that images the printed material S, which in turn are ascribable to factors such as: the imaging environment (e.g., temperature), mounting accuracy of the imaging device with respect to the printing apparatus, and/or fluctuations in the position at which the printed material S is imaged (fluctuations in the distance between the imaging device and the printed material S). In particular, changes in the distance between the imaging device and the printed material S may be caused by recoil and like actions of the printed material S during the transportation thereof, since the printed material S is read by the printing apparatus while travelling, thereby making it impossible to obtain stable positioning. Errors in the estimated patch positions due to such fluctuations in the imaging magnification associated with the imaging device become more likely to occur as the total number of reference marks rm is decreased. The magnitude of the errors in the estimated patch positions become greater for patches which are located farther away from the reference marks rm. Due to such errors, the patch processing device in the printing apparatus may end up measuring color density information at positions away from the patch centers, resulting in inaccurate measurements of the color density information.
Therefore, an object of the present invention is to provide a patch measurement device which is capable of accurately detecting the positions of patches composing a control strip even if the imaging magnification associated with an imaging device fluctuates with respect to a printed material, such a patch measurement device being provided in a printing apparatus.
The present invention has the following features to attain the object mentioned above.
A first aspect of the present invention is directed to a patch measurement device provided in a printing apparatus for detecting a patch position representing a position of a patch in a control strip printed on paper by the printing apparatus, wherein, the control strip and a reference mark are printed on the paper with a prescribed relative distance between the reference mark to the patch, and printed-image data representing the control strip and the reference mark printed on the paper are generated through imaging by an imaging device provided in the printing apparatus, the patch measurement device comprising: a storage section for storing the printed-image data sent from the imaging device; a mark detection section for detecting the reference mark based on the printed-image data stored in the data storage section; a correction section for calculating as a correcting coefficient a ratio between a printed-image data length and a predetermined known length of a position corresponding to the printed-image data length, wherein the printed-image data length is a length on the printed-image data calculated based on the reference mark detected by the mark detection section; and a position detection section for detecting the patch position with respect to the reference mark on the printed-image data, based on the correcting coefficient calculated by the correction section and the prescribed relative distance.
Thus, according to the above structure of the present invention, a ratio between printed-image data and a predetermined known value is calculated as a correcting coefficient, by utilizing a reference mark. In the calculation of a patch position with respect to a reference mark in the printed-image data, a correcting coefficient is applied to a prescribed relative position of the patch, whereby an accurate patch position can be calculated even if the imaging magnification for the printed-image data fluctuates. Therefore, patch positions can be accurately measured even if the imaging magnification (associated with the imaging device which images a printed material) for the printed-image data fluctuates due to reasons associated with the imaging environment (e.g., temperature), mounting accuracy of the imaging device with respect to the printing apparatus, and/or fluctuations in the position at which the printed material is imaged (fluctuations in the distance between the imaging device and the printed material). In particular, stable position measurement is possible even if the distance between the imaging device and the printed material changes due to recoil and like actions of the printed material, which may occur when the printed material is read by the printing apparatus during the transportation of the printed material.
In one embodiment, a plurality of said reference marks is printed on the paper, the mark detection section detects the plurality of reference marks based on the printed-image data stored in the data storage section, the printed-image data length is calculated based on a distance between the plurality of reference marks detected by the mark detection section, and the predetermined known length is the distance between the plurality of reference marks. Thus, a correcting coefficient is calculated based on a distance between reference marks, whereby an accurate correcting coefficient can be calculated. By utilizing a line connecting the reference marks as a reference direction for printing, it becomes possible to extract an accurate patch from the printed-image data even if the printed material is imaged by the imaging device with an offset from the predetermined position. In another embodiment, the position detection section detects the patch position with respect to the reference mark on the printed-image data based on one of the plurality of reference marks that is closest to the patch. As a result, a patch position on the printed-image data can be more accurately detected.
In one embodiment, the control strip and the reference mark are printed on the paper based on image-to-print data representing an image to be printed, and the known length and the relative distance are described in the image-to-print data. In this case, xe2x80x9cimage-to-print dataxe2x80x9d, i.e., data representing an image to be printed on paper by the printing apparatus, a known length necessary for detecting a patch position and a prescribed relative distance of the patch are easily obtained, thereby making it easy to obtain theoretically-known values. In another embodiment, the known length is set by previously measuring the length of a printed image corresponding to the printed-image data length printed on the paper. In this case, by actually measuring a image printed on paper by the printing apparatus, theoretically-known values can be easily obtained.
In still another embodiment, a plurality of said reference marks are printed on the paper, the imaging device comprises a first and second imaging units, the first and second imaging units generate respectively different first and second read-out image data, each containing at least two said reference marks, such that the printed-image data is generated by synthesizing the first and second read-out image data, the mark detection section detects the plurality of reference marks based on the printed-image data stored in the data storage section, the printed-image data length comprises: a first printed-image data length which is calculated based on a distance between the at least two reference marks contained in a region of the printed-image data generated from the first read-out image data; and a second printed-image data length which is calculated based on a distance between the at least two reference marks contained in a region of the printed-image data generated from the second read-out image data, the known length comprises: a predetermined first known length corresponding to the first printed-image data length; and a predetermined second known length corresponding to the second printed-image data length, the correcting coefficient comprises: a first correcting coefficient which is calculated based on a ratio between the first printed-image data length and the first known length; and a second correcting coefficient which is calculated based on a ratio between the second printed-image data length and the second known length, and the position detection section is operable to: if the patch is within the region of the printed-image data generated from the first read-out image data, detect the patch position with respect to the reference mark on the printed-image data based on the first correcting coefficient and the prescribed relative distance for the patch, and if the patch is within the region of the printed-image data generated from the second read-out image data, detect the patch position with respect to the reference mark on the printed-image data based on the second correcting coefficient and the prescribed relative distance for the patch. In the case where the imaging device is composed of a plurality of imaging units, fluctuations in the imaging magnification for the printed-image data may occur for each imaging unit. However, since two magnification correcting coefficients k are calculated for each of the two regions of the printed-image data which are imaged by the two imaging units (such two regions being generated out of the read-out image data), correction can be performed in accordance with the respective fluctuation in imaging magnification.
The patch measurement device may further comprise a color density measurement section for measuring color density of the patch whose position has been detected by the position detection section. Thus, the position detection section can calculate an accurate patch position, whereby a patch measurement device which can measure accurate color density information is provided.
A second aspect of the present invention is directed to a printing apparatus for printing an image to be printed, a control strip, and a reference mark on paper, comprising: a prepressing mechanism for receiving external image-to-print data representing the image to be printed, and forming the image to be printed, the control strip, and the reference mark on a printing plate, based on the image-to-print data; a printing mechanism for applying at least ink on the printing plate fed from the prepressing mechanism, and transferring the image to be printed, the control strip, and the reference mark from the printing plate having the ink applied thereto onto the paper; an imaging device for imaging, within the printing mechanism, a portion of the paper where at least the control strip and the reference mark are printed, thereby generating printed-image data; and a patch measurement device for, based on the printed-image data generated by the imaging device, detecting a patch position representing a position of a patch in the control strip with respect to the reference mark and measuring color density of the patch, wherein a prescribed relative distance exists between the reference mark to the patch, the patch measurement device comprising: a storage section for storing the printed-image data sent from the imaging device; a mark detection section for detecting the reference mark based on the printed-image data stored in the data storage section; a correction section for calculating as a correcting coefficient a ratio between a printed-image data length and a predetermined known length of a position corresponding to the printed-image data length, wherein the printed-image data length is a length on the printed-image data calculated based on the reference mark detected by the mark detection section; and a position detection section for detecting the patch position with respect to the reference mark on the printed-image data, based on the correcting coefficient calculated by the correction section and the prescribed relative distance; and a color density measurement section for measuring the color density of the patch whose position has been detected by the position detection section, wherein the printing mechanism adjusts the amount of ink to be applied to the printing plate based on the color density of the patch having been measured by the patch measurement device.
Thus, according to the above structure of the present invention, the aforementioned effects according to the first aspect of the present invention can be attained in a printing apparatus.
In one embodiment, a plurality of said reference marks are printed on the paper, and the correction section calculates the printed-image data length based on a distance between the reference marks. In another embodiment, the position detection section detects the patch position with respect to the reference mark on the printed-image data based on one of the plurality of reference marks that is closest to the patch.
A third aspect of the present invention is directed to a patch measurement method for detecting a patch position representing the position of a patch in a control strip printed on paper, wherein the control strip and a reference mark are printed on the paper with a prescribed relative distance between the reference mark to the patch, the method comprising: a storage step of storing printed-image data representing the control strip and the reference mark printed on the paper; a mark detection step of detecting the reference mark based on the printed-image data stored by the data storage step; a correction step of calculating as a correcting coefficient a ratio between a printed-image data length and a predetermined known length of a position corresponding to the printed-image data length, wherein the printed-image data length is a length on the printed-image data calculated based on the reference mark detected by the mark detection step; and a position detection step of detecting the patch position with respect to the reference mark on the printed-image data, based on the correcting coefficient calculated by the correction step and the prescribed relative distance.
Thus, according to the above structure of the present invention, a ratio between printed-image data and a predetermined known value is calculated as a correcting coefficient, by utilizing a reference mark. In the calculation of a patch position with respect to a reference mark in the printed-image data, a correcting coefficient is applied to a prescribed relative position of the patch, whereby an accurate patch position can be calculated even if the imaging magnification for the printed-image data fluctuates. Therefore, patch positions can be accurately calculated even if the imaging magnification for the printed-image data which is obtained by imaging a printed material fluctuates due to reasons associated with the imaging environment (e.g., temperature), the imaging position, and/or fluctuations in the position at which the printed material is imaged. In particular, stable position calculation is possible even if the in the position at which the printed material is imaged fluctuates due to recoil and like actions of the printed material, which may occur when the printed material is read during the transportation thereof.
In one embodiment, a plurality of said reference marks are printed on the paper, the mark detection step detects the plurality of reference marks based on the printed-image data stored by the data storage step, the printed-image data length is calculated based on a distance between the plurality of reference marks detected by the mark detection step, and the predetermined known length is the distance between the plurality of reference marks. In another embodiment, the position detection step detects the patch position with respect to the reference mark on the printed-image data based on one of the plurality of reference marks that is closest to the patch.
In another embodiment, a plurality of said reference marks are printed on the paper, the printed-image data stored by the data storage step represents the control strip and the plurality of reference marks printed on the paper, the printed-image data being generated by synthesizing respectively different first and second read-out image data, each containing at least two said reference marks, the mark detection step detects the plurality of reference marks based on the printed-image data stored by the data storage step, the printed-image data length comprises: a first printed-image data length which is calculated based on a distance between the at least two reference marks contained in a region of the printed-image data generated from the first read-out image data; and a second printed-image data length which is calculated based on a distance between the at least two reference marks contained in a region of the printed-image data generated from the second read-out image data, the known length comprises: a predetermined first known length corresponding to the first printed-image data length; and a predetermined second known length corresponding to the second printed-image data length, the correcting coefficient comprises: a first correcting coefficient which is calculated based on a ratio between the first printed-image data length and the first known length; and a second correcting coefficient which is calculated based on a ratio between the second printed-image data length and the second known length, and the position detection step comprises: if the patch is within the region of the printed-image data generated from the first read-out image data, detecting the patch position with respect to the reference mark on the printed-image data based on the first correcting coefficient and the prescribed relative distance for the patch, and if the patch is within the region of the printed-image data generated from the second read-out image data, detecting the patch position with respect to the reference mark on the printed-image data based on the second correcting coefficient and the prescribed relative distance for the patch.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.